Living in New York, it’s easy to forget that the ocean is right on our doorsteps. This isn’t Miami with its beaches or Venice with its canals or New Orleans with its history of storms and floods. New York has always been a supremely self-involved city—this famous magazine cover pretty much sums it up—and though Manhattan is an island, it’s one that has its eyes turned inward, not out toward the water that rings it.

Hurricane Sandy ended that illusion last year. The storm surge flooded tunnels, subway lines and apartment buildings; swamped power lines and transformers caused a blackout over much of Manhattan that lasted for days. Altogether Sandy cost the city of New York some $19 billion in public and private losses, nearly all of it due to the water. Sandy wasn’t even that powerful a storm, its winds barely ranking as a category 1 when it made landfall along the East Coast last October. What it had was something any New Yorker who’s hunted for apartments could appreciate—location, location, location—hitting the biggest city in America and flooding it with all that forgotten coastal water.

For coastal cities like New York, Hurricane Sandy was a coming attraction for what is likely to be a very wet and destructive future. According to leaked drafts of the forthcoming new report from the Intergovernmental Panel on Climate Change (IPCC), scientists believe that sea level could rise by more than three feet by the end of the century is carbon emissions keep growing at a runaway pace. And a new study published in the journal Nature Climate Change laid out the enormous flood losses that major coastal cities could face in the future. Average global flood losses could rise from approximately $6 billion per year in 2005 to $60 to $63 billion per year by 2050, thanks to population and economic growth along the coasts and the multiplying effect of climate change-driven sea level rise. As Robert Nicholls, a professor of coastal engineering at the University of Southampton in Britain and a co-author of the study, put it in a statement: “There is a pressing need to start planning how to manage flood risk now.”

The Nature Climate Change study looked at both present and projected future flood losses in the 136 largest coastal cities in the world, looking at their financial risks both in absolute terms—taking into account protections like sea walls and dikes—and as a percentage of the city’s GDP. The cities ranked as most at risk today range from Guangzhou in southern China to Mumbai in India to, yes, New York City. What those cities tend to have in common is high wealth and population levels and relatively little flooding protection. (By contrast, Dutch cities like Amsterdam or Rotterdam—which are extremely flood-prone geographically—aren’t found on the list because the Netherlands government has invested heavily in coastal protection.) Three American cities—Miami, New York and New Orleans—are responsible for 31% of the total losses across the 136 cities surveyed in 2005. When it comes to losses as a percentage of total city GDP—which gives the very richest cities like New York an advantage—Guangzhou, New Orleans and Guayaquil in Ecuador are most at risk.

The situation changes a bit in 2050. The study assumed that climate change will lead sea levels to rise 0.65 to 1.3 ft. by 2050, with some cities facing additional sea level rise because of local subsidence—literally, the earth sinking. Developing cities like Guangzhou, Mumbai and Shenzhen face the biggest risks, though Miami and New York rank highest among cities in developed nations. If no improvements are made in flood defenses, the study estimates that the world could be facing as much as $1 trillion or more per year in losses. Now, that number is the worst of the worst case, assuming that cities do absolutely nothing to protect themselves from sea level rise, suffer major floods and then pay to immediately rebuild everything they lost. But even assuming improvements in coastal defenses, potential losses will increase significantly, thanks to the risk of bigger floods and more immediately, a huge increase in the number of people and the value of property along the coasts.

That second bit is important. It’s vital for governments to gain a better understanding of flooding risks from global warming—and sea level rises of the sort apparently projected by the IPCC will endanger major world cities. But the most immediate threat is the sheer increase in people—and their property—put in harm’s way in coastal cities. In the U.S. 87 million people now live along the coast, up from 47 million people in 1960, and globally six of the world’s 10 largest cities are on the coast. Of the $60 to $63 billion in flood risk the Nature Climate Change study estimates the world’s cities will face by 2050, $52 billion is due to economic and population growth—the rest is due to sea level rise and land use change.

That doesn’t mean that climate change-amplified floods and storms don’t present a danger to coastal cities—or that we don’t need to worry about reducing carbon emissions. But the numbers don’t lie—the single biggest increase in the risk from flooding comes from putting people and property in places where floods have always been likely to happen. As Sandy showed, coastal cities are at risk from major flooding right now if a storm should hit at the wrong place at the wrong time. It’s location, location, location—which is why it’s so important to spend money now to improve coastal defenses. We don’t have to wait for climate change to come.